A turbocharger forms part of an engine, and comprises a turbocharger shaft driven by a turbine that rotates in response to exhaust gases from the engine. The principal purpose of the turbocharger is to compress gases using a compressor for introduction into the engine cylinders (called “boost”).
Multiple turbochargers can be implemented in a sequential arrangement. This can reduce the time required to bring the turbocharger to a speed where it can function effectively, known as turbo lag. When the boost pressure is too high, it can be reduced by causing the exhaust gases to bypass one of the turbochargers. The exhaust gases may instead be diverted through a wastegate. For example, in a two-stage air system, the wastegate may be situated in the High Pressure (HP) turbocharger stage, which is the stage closer to the engine, although it can also be situated in the Low Pressure (LP) turbocharger stage or in both turbocharger stages.
The operation of such systems may be explained using an example. Referring to FIG. 1, there is shown an example torque-speed characteristic for an existing engine using measured data. The line in this drawing may specify the maximum torque attainable as a function of engine speed.
In this example, the engine generally runs at constant speed with sudden sharp changes in the demanded load. Plotting this engine operation on the torque-speed curve, points T1a, T2a and T3a represent what happens to engine speed and load at three time points during a transient event. In this example, the transient event is when the demanded load drops to 60% of the starting torque. The starting torque is shown at T1a, the changed torque is shown at T2a and the final system equilibrium torque is shown at T3a.
Referring next to FIG. 2, there is illustrated examples of relevant parameters of an existing turbocharger system, when operated with the engine described by FIG. 1. These may be effectively considered as real time histories of the relevant parameters for this sudden drop in demanded torque event. The abscissa may represent approximately 10 seconds of operation.
In this example, the engine speed is more or less constant, remaining within +/−5% of the rated speed. Then, after reaching a peak at T1a, the load torque is suddenly reduced. The engine power follows the torque since the speed is approximately constant.
The boost pressure being delivered at T1a exceeds that required to produce the new, lower, demanded torque. To prevent the engine accelerating due to the lower absorbed torque, the boost pressure is desirably reduced. The drop off in torque may be sensed through the rise in boost pressure.
The wastegate starts to open as a result of this sensed rise in boost pressure (as illustrated), causing the boost pressure to drop. In this way, the specific energy of the exhaust flow reaching the turbine is thus reduced and less energy is transferred to the compressor. The compressor speed and boost pressure start to decrease as the wastegate is opened. It should be noted that the turbocharger speed is directly related to boost pressure and will likely have the same trend as the boost pressure.
Between time T2a and time T3a, the sensed boost pressure adjusts the wastegate opening until the engine torque matches the demand. By time T3a the specific energy in the exhaust flow matches that required for the desired boost level and so the wastegate closes.
An electric turbo assist (ETA) turbocharger also generates electrical energy through rotation of the shaft. The generated energy can be stored in batteries, used in auxiliary electrical systems or fed to a motor connected to the engine crankshaft to improve engine response. The ETA system provides an additional mechanism to recover energy that might otherwise be lost where the energy in the exhaust gases exceeds what is needed to drive the compressor.
Applying ETA technology to a multiple turbocharger system poses a number of difficulties, due to the system complexity. JP-2005-009315 shows such a two-stage turbocharger system with dynamo-electric machines coupled to both stages and with a by-pass valve able to cause exhaust gases to by-pass the HP stage. The by-pass valve and dynamo-electric machines are controlled on the basis of the engine speed, engine load and whether the engine is decelerating. In some cases, opening the by-pass valve is considered appropriate. However, the energy efficiency of this implementation is limited and large losses can be incurred on the HP stage at high speeds.
WO-2010/039197 relates to a hydrogen fuelled powerplant including an internal combustion engine with an afterburner in the exhaust section and a two-stage turbocharger. In one embodiment, turbine generators are coupled to both stages of the turbocharger. The turbine generators are configured to remove excess energy resulting from the afterburner operation. Low-pressure and high-pressure wastegates are used to bleed off some of the pressurized exhaust when the engine speed or load changes and the compression capability of the system detrimentally overwhelms the engine requirements.